1. Field of the Invention
This invention relates to an encoder for detecting the absolute position on a topographic surface of an element.
2. Description of the Related Art
Position encoders are used to accurately determine a position on a surface of an element of a device or system. Position encoders may measure relative or absolute position. A relative position encoder provides position information indicating the change from a prior position, while an absolute position encoder provides absolute position information indicating a specific position regardless of prior position. Position encoders are used in automated manufacturing, gimbaled systems, and elsewhere when accurate positional information is desired. In gimbaled-camera systems, for example, absolute position encoders may be used for accurate line-of-sight reconstruction in guidance.
Some conventional position encoders place encoder tracks (patterns of alternating dark and light surfaces) on the surface of the element. Detectors sense light reflected off the pattern due to relative motion of the element and detector. A simple count of the alternating pattern may provide relative motion. Absolute position may be determined via an initial calibration.
Some position encoders use separate encoder tracks for each bit of a Grey code, in which only one bit of the code changes at a time. Detectors are used to detect which bit changes to determine a position. One problem with this arrangement is that higher resolution requires a high number of separate encoder tracks. Another problem is that this arrangement is highly sensitive to contamination, which results in erroneous position information.
U.S. patent pub. No. 2004/0173735 entitled “Absolute Incremental Position Encoder and Method” describes a position encoder that detects bit-width transitions from a sequence having a plurality of unique subsequences. In embodiments, the position encoder may use a single track encoded with a pattern of bit-widths in accordance with the sequence. The sequence may be a pseudo-random noise (PRN) sequence or other sequence having unique subsequences. In one embodiment, sensors detect transitions between the bit-widths as the track moves to provide in-phase and quadrature-phase pick-off signals. When a PRN sequence is used having a length of 2N bits, the position of the track may be an absolute position when the number of transitions between the bit-widths detected by the sensors is at least N. The position may be an incremental position when the number of transitions between bit-widths detected by the sensors is less than N. In one embodiment, each bit-width encoded on the track has either a first width or a second width determined by the sequence. The first width may represent the “ones” in the sequence and the second width may represent the “zeroes” in the sequence. The pattern on the track may be a pattern of alternating dark and light portions having the bit-widths encoded in accordance with bits of the sequence, and the first- and second sensors may be optical sensors positioned to have overlapping fields of view.